Ultra-pure fluids including de-ionized water are frequently used for processing of sensitive materials such as semi-conductor substrates. The susceptibility to contamination of the sensitive materials during the manufacturing process is a significant problem faced by manufacturers. The sensitive materials under process are often in direct contact with the ultra-pure fluids. Hence, contamination of the ultra-pure fluids often results in contamination of the materials under process. Various components of these manufacturing systems, such as flow meters and controllers have been designed to reduce the contamination of the ultra-pure fluids (and therefore the sensitive materials under process) by reducing the harboring of foreign particles, and by preventing the growth of bacteria through elimination or reduction of stagnant flow regions within the fluid delivery system.
Flow meters and controllers typically require the measurement of fluid pressure on either side of an orifice in the fluid flow path. The differential in measured pressure is used to calculate the fluid flow rate. Conventional fluid pressure measurements are obtained by tapping the flow line at the desired location and putting the tap in fluid communication with a pressure sensor. To prevent the formation of stagnant flow regions that harbor particulates and facilitate bacterial growth, pressure tap geometries having a small “aspect ratio” (length over diameter) are used. A tap geometry that is shallow with respect to the penetration diameter tends to be swept clean by the process flow, and is less likely to promote bacterial growth or harbor particulates. Further, devices for ultra-pure process flows are typically flushed with process fluid flow for a period of time upon initial installation, during which time the actual process is out of operation. It is desirable that regions out of the direct fluid flow be kept to a minimum to enable quick flushing and removal of contaminants. Hence, conventional design favors a pressure tap with a pressure sensor that is closely coupled to the wall of the flow passage. Preferably, the sensing face of the pressure sensor is flush with the wall of the flow passage.
Flow controllers, such as those manufactured by Entegris NT (owner of the present application) and disclosed in U.S. Pat. No. 6,578,435 hereby fully incorporated herein by reference, commonly use ceramic pressure sensors that are in direct contact with the process medium. These ceramic sensors, however, are not well suited for contact with caustic fluids. Direct contact of the pressure sensor with the process medium is preferred over arrangement wherein an isolating diaphragm is interposed between the sensor and the process fluid in part because direct contact enhances the responsiveness of the measurement, and reduces other secondary effects (e.g. hysteresis) associated with isolation techniques. As a result, sapphire or other such inert materials are often used with caustic fluids. Such a sensor is disclosed, for example, in U.S. Pat. No. 6,612,175, which is owned by the owner of the present invention and is hereby fully incorporated herein by reference.
Applicant has discovered, however, that sapphire sensors are susceptible to bias errors and signal noise due to thermal effects when they are positioned with the sensing face flush with the wall of the flow passage. Particularly in integrated flow controllers wherein a motor operated valve is disposed in the same body with the flow metering sensors, it is believed that heat from the motor is conducted both through the valve body and the fluid itself to reach the sensors. The sensor located closest to the valve motor will be at a generally higher temperature than the sensor located further away due to conduction through the valve body. Further, differential temperatures may exist along the length of the portion of the sensor disposed in the valve body due to conduction through the valve body. Heat may be conducted through the fluid, even upstream against the flow direction in very low fluid flow applications, for example lower than about 250 mL/min. The thermal resistance of the fluid may result in differential fluid temperatures at the sensor faces causing a bias. Further, flow turbulence and differential heating at the fluid to valve body interface may result in pockets of fluid at differing temperatures. The pockets of fluid at differing temperatures may contact the sensor faces, resulting in rapid output signal variations.
What is needed in the industry are flow metering and controlling devices wherein sapphire sensors may be placed in direct contact with a process fluid while avoiding undesirable signal bias and noise from thermal effects, while simultaneously avoiding regions of stagnant fluid harboring particulates and facilitating bacterial growth.